In physical cosmology, galaxy filaments (subtypes: supercluster complexes, galaxy walls, and galaxy sheets) are the largest known structures in the universe. They are massive, thread-like formations, with a typical length of 50 to 80 megaparsecs h−1 (163 to 261 million light-years) that form the boundaries between large voids in the universe. Filaments consist of gravitationally bound galaxies. Parts wherein many galaxies are very close to one another (in cosmic terms) are called superclusters.